Marks Mac flavors_test_macos to be unflaky (#140647)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Mac flavors_test_macos"
}
-->
The test has been passing for [50 consecutive runs](https://data.corp.google.com/sites/flutter_infra_metrics_datasite/flutter_check_test_flakiness_status_dashboard/?p=BUILDER_NAME:%22Mac%20flavors_test_macos%22).
This test can be marked as unflaky.

.ci.yaml

@@ -3384,7 +3384,6 @@ targets:
       - .ci.yaml
 
   - name: Mac flavors_test_macos
-    bringup: true
     presubmit: false
     recipe: devicelab/devicelab_drone
     timeout: 60